Recently, as automobile performances have been improved and road network has developed, steering stability of tires is also required. In order to obtain superior steering stability of tires, it is necessary to improve rigidity of a bead apex and there has been developed rubber compositions for a bead apex comprising a large quantity of carbon black.
However, although high rigidity can be obtained by using fillers, there are also disadvantages that heat is easily generated during tire running because of increase in loss tangent (tan δ), durability is damaged by thermal fatigue and rolling resistance of a tire is increased (rolling resistance property is reduced).
Further, environmental problems have been recently emphasized and regulations on CO2 emission have been made more rigorous. Further, since petroleum resources are finite and supply quantity thereof has been reduced year by year, petroleum price is predicted to surge in future and there is a limit in use of raw materials derived from petroleum resources such as carbon black. Consequently, assuming a case of depletion of petroleum in future, it is necessary to use resources other than petroleum such as a natural rubber, silica and calcium carbonate for a rubber composition for a bead apex. However, in such a case, there is a problem that equal or better properties such as rigidity of a bead apex cannot be obtained compared with the case of conventionally used petroleum resources.
JP2003-63206A discloses raw materials for a tire to cope with the case where depletion of petroleum is assumed to occur, but there is not disclosed a rubber composition for a bead apex that reduces rolling resistance adequately and is superior in steering stability.